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POHaskell Aeson with HTTP Request
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  1. POHaskell Aeson with HTTP Request
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    copied!<p>I'm a complete newbie to Haskell and having some trouble parsing JSON from a url. I have managed to code the parsing sides of things and have tested with Strings and they work fine. The trouble I'm getting is when I start working with IO. </p> <p>I have the following code :</p> <pre><code>data Movie = Movie { adult :: Bool, backdrop_path :: Maybe String, id :: Int, original_title :: String, release_date :: String, poster_path :: Maybe String, popularity :: Int, title :: String, vote_average :: Double, vote_count :: Int } deriving (Show) data MovieList = MovieList {getMovies ::[Movie]} deriving (Show) instance FromJSON Movie where parseJSON (Object v) = Movie &lt;$&gt; (v .: "adult") &lt;*&gt; (v .:? "backdrop_path") &lt;*&gt; -- Optional (v .: "id") &lt;*&gt; (v .: "original_title") &lt;*&gt; (v .: "release_date") &lt;*&gt; (v .:? "poster_path") &lt;*&gt; -- Optional (v .: "popularity") &lt;*&gt; (v .: "title") &lt;*&gt; (v .: "vote_average") &lt;*&gt; (v .: "vote_count") instance FromJSON MovieList where parseJSON (Object o) = MovieList &lt;$&gt; o .: "results" parseJSON _ = mzero movieAPIRequest :: String -&gt; IO String movieAPIRequest movieURI = do resp &lt;- simpleHTTP request case resp of Left x -&gt; return $ "Error connecting: " ++ show x Right r -&gt; case rspCode r of (2,_,_) -&gt; return $ rspBody r -- Request Fulfilled _ -&gt; return $ show r -- Request Failed where request = Request {rqURI = uri, rqMethod = GET, rqHeaders = [], rqBody = ""} uri = fromJust $ parseURI movieURI convertToByteString s = BS.pack s main = do response &lt;- movieAPIRequest url decoded &lt;- decode (convertToByteString response):: Maybe MovieList return $ decoded </code></pre> <p>I can't get the main to work. I want to automatically retrieve JSON from a url. The <code>movieAPIRequest</code> gives me the body of the request (JSON) as <code>IO String</code>. <code>convertToByteString</code> takes a String and converts to Data.ByteString.Lazy.Char8.ByteString as the <code>decode</code> function in Aeson takes a bytestring as an argument. With the above code I'm getting the following error:</p> <p>[1 of 1] Compiling MovieDataType ( MovieDataType.hs, interpreted )</p> <pre><code>MovieDataType.hs:62:20: Couldn't match type `Maybe' with `IO' Expected type: IO MovieList Actual type: Maybe MovieList In a stmt of a 'do' block: decoded &lt;- decode (convertToByteString response) :: Maybe MovieList In the expression: do { response &lt;- movieAPIRequest url; decoded &lt;- decode (convertToByteString response) :: Maybe MovieList; return $ decoded } In an equation for `main': main = do { response &lt;- movieAPIRequest url; decoded &lt;- decode (convertToByteString response) :: Maybe MovieList; return $ decoded } Failed, modules loaded: none. </code></pre> <p>I've tried fixing it but keep getting different that I can't get my head around. For example this one is telling its expecting IO MovieList but decode should return a Maybe MovieList.</p>
 

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